August 26, 1920] 



NATURE 



821 



of years later'* Dr. Marie Lebour published a detailed 

 account of her work at Plymouth on the food of young 

 fishes, proving that certain fish undoubtedly do prefer 

 certain planktonic food. 



These Crustacea of the plankton feed upon smaller 

 and simpler organisms — the Diatoms, the Peridinians, 

 and the Flagellates — and the fish themselves in their 

 youngest post-larval stages are nourished by the same 

 minute forms of the plankton. Thus it appears that 

 our sea-fisheries ultimately depend upon the living 

 plankton, which no doubt, in its turn, is affected by 

 hydrographic conditions. A correlation seems to be 

 established between the Cornish pilchard fisheries and 

 periodic variations in the physical characters (probably 

 the salinity) of the water of the English Channel 

 between Plymouth and Jersey." Apparently a 

 diminished intensity in the Atlantic current corre- 

 sponds with a diminished fishery in the following 

 summer. Possibly the connection in these cases is 

 through an organism of the plankton. 



It is only a comparatively small number of different 

 kinds of organisms — both plants and animals — that 

 make up the bulk of the plankton that is of real 

 importance to fish. One can select about half a dozen 

 species of Copepoda which constitute the greater part 

 of the summer zoo-plankton suitable as food for larval 

 or adult fishes, and about the same number of generic 

 types of Diatoms which similarly make up the bulk 

 of the available spring phyto-plankton year after year. 

 This fact gives great economic importance to the 

 attempt to determine with as much precision as pos- 

 sible the times and conditions of occurrence of these 

 dominant factors of the plankton in an average year. 

 An obvious further extension of this investigation is 

 an inquiry into the degree of coincidence between the 

 times of appearance in the sea of the plankton 

 organisms and of the young fish, and the possible 

 effect of any marked absence of correlation in time 

 and quantity. 



Just before the war the International Council for 

 the Exploration of the Sea " arrived at the conclusion 

 that fishery investigations indicated the probability 

 that the great periodic fluctuations in the fisheries are 

 connected with the fish-larvae being developed in great 

 quantities only in certain years. Consequently they 

 advised that ' plankton work should be directed 

 primarily to the question whether these fluctuations 

 depend up>on differences in the plankton production in 

 different years. It was then proposed to begin sys- 

 tematic investigation of the fish-larvae and the plankton 

 in spring and to determine more definitely the food 

 of the larval fish at various stages. 



About the same time Dr. Hjort " made the interest- 

 ing suggestion that possibly the great fluctuations in 

 the number of young fish observed from year to year 

 may not depend wholly upon the number of eggs pro- 

 duced, but also upon the relation in time between the 

 hatching of these eggs and the appearance in the 

 water of the enormous quantity of Diatoms and other 

 plant plankton upon which the larval fish, after the 

 absorption of their yolk, depend for food. He points 

 out that if even a brief interval occurs between the 

 time when the larvae first require extraneous nourish- 

 ment and that when such food is available, it is 

 highly probable that an enormous mortality would 

 result. In that case even a rich spawning season 

 might yield but a poor resiilt in fish in the commercial 

 fisheries of successive years for some time to come. 

 So that, in fact, the numbers of a year-class may 

 depend not so much upon a favourable spawning 

 season as upon a coincidence between the hatching of 



18 Journ. Mar. Biol. Assoc, May, 1918. 



17 See E. C. Jer, " Hydrography of the Engl'sh Channel," 1904-17. 



18 Rapports et Proc. Verh., vol. xix., December, 1915. 



19 Ibid., vol. XX., 1914, p. 204. 



the larvae and the presence of abundance of phyto- 

 plankton available as food." 



The curve for the spring maximum of Diatoms cor- 

 responds in a general way with the curve representing 

 the occurrence of pelagic fish-eggs in our seas. But 

 is the correspondence sufficiently exact and constant 

 to meet the needs of the case? The phyto-plankton 

 may still be relatively small in amount during Februarv- 

 and part of March in some years, and it is not easy 

 to determine exactly when, in the open sea, the fish- 

 eggs have hatched out in quantity and the larvae have 

 absorbed their food-yolk and started feeding on 

 Diatoms. 



If, however, we take the case of one important fish 

 —the plaice— we can get some data from our hatching 

 experiments at the Port Erin Biological Station, which 

 have now been carried on for a period of seventeen 

 years. An examination of the hatchery records for 

 these years in comparison with the plankton records 

 of the neighbouring sea, which have been kept sys- 

 tematically for the fourteen years from 1907 to 1920 

 inclusive, shows that in most of these years the 

 Diatoms were present in abundance in the sea a few 

 days at least before the fish-larvae from the hatcherv 

 were set free, and that it was only in four year's 

 (1908, 1909, 19 13, and 19 14) that tnere was apparently 

 some risk of the larvae finding no phyto-plankton food 

 or very little. The evidence so far seems to show that 

 if fish-larvae are set free in the sea so late as March 20 

 they are fairly sure of finding suitable food *' ; but if 

 they are hatched so early as February they run some 

 chance of being starved. 



But this does not exhaust the risks to the future 

 fishery. C. G. Joh. Petersen and Boysen-Jensen in 

 their valuation of the Limf jord "* have shown that in 

 the case not only of some fish, but also of the larger 

 invertebrates on which they feed, there are marked 

 fluctuations in the number of young produced in 

 different seasons, and that it is only at intervals of 

 years that a really large stock of young is added to 

 the population. 



The prospects of a year's fishery may, therefore, 

 depend primarily upon' the rate of spawning of the 

 fish, affected, no doubt, by hydrographic and other 

 environmental conditions; secondarily, upon the 

 presence of a sufficient supply of phyto-plankton in 

 the surface-layers of the sea at the time when the 

 fish-larvae are hatched, and that, in its turn, depends 

 upon photosynthesis and physico-chemical changes in 

 the water; and, finally, upon the reproduction of the 

 stock of molluscs or worms at the bottom which con- 

 stitute the fish-food at later stages of growth and 

 development. 



The question has been raised in recent years : Is 

 there enough plankton in the sea to provide sufficient 

 nourishment for the larger animals, and especially 

 for those fixed forms, such as sponges, that are sup- 

 posed to feed by drawing currents of plankton-laden 

 water through the body? In a series of remarkable 

 papers from 1907 onwards Putter and his followers 

 put forward the views : (i) that the carbon require- 

 ments of such animals could not be met by the 

 amount of plankton in the volume of water that could 

 be passed through the body in a given time, and 

 (2) that sea-water contained a large amount of dis- 

 solved organic carbon compounds which constitute 

 the chief, if not the only, food of a large number of 

 marine animals. These views have given rise to 



20 For the purpose of this argument we may include in " phyto- 

 plankton " the various group? of Flagellata and i ther minute organisms 

 which may be present with the Diatoms. 



*1 All dates and statements as to occurrence refer to the Irish Sea round 

 the south end of the Isle of Man. For further details see Report Lanes 

 SeJi-Fi«h. Lab. for 1919. 



12 Report of Danish Bio'. Station for 1919. 



NO. 2652, VOL. 105] 



